SiAlON phosphors are a kind of oxynitride phosphors including chemical elements such as Si, Al, O and N, and it has been known that there are two kinds of the SiAlON phosphors having different crystal structures: α-SiAlON phosphor and β-SiAlON phosphor. The α-SiAlON phosphor is described in non-patent reference 1, and the α-SiAlON phosphor and the use of LED using the same are described in patent references 1 to 4. Also, the β-SiAlON phosphor is described in patent reference 5, and the β-SiAlON phosphor and the use of LED using the same are described in patent reference 6.    [Non-patent reference 1] J. W. H. van Krebel “On new rare earth doped M-Si—Al—O—N materials”, Tu Eindhoven The Netherland, P 145-161(1998)    [Patent reference 1] Japanese Laid-Open Patent Publication No. 2002-363554    [Patent reference 2] Japanese Laid-Open Patent Publication No. 2003-336059    [Patent reference 3] Japanese Laid-Open Patent Publication No. 2004-238505    [Patent reference 4] Japanese Laid-Open Patent Publication No. 2007-31201    [Patent reference 5] Japanese Laid-Open Patent Publication No. Sho60-206889    [Patent reference 6] Japanese Laid-Open Patent Publication No. 2005-255895
α-SiAlON has a crystal structure having a unit structure represented by Formula: Si12-(m+n)Al(m+n)OnN16-n and having two sites formed therein. Metal ions, such as Ca2+ having a relatively smaller ion radius may be dissolved into the sites, and the metal ion-dissolved α-SiAlON may be represented by Formula: Mm/vSi12-(m+n)Al(m+n)OnN16-n:Eu (wherein, M is a metal ion, v represents a valence of the metal ion). It has been known that α-SiAlON in which Ca and an activator Eu are dissolved is a yellow-emitting phosphor, as described in the non-patent reference 1 and the patent reference 1. The α-SiAlON phosphor has an excitation wavelength band ranging from ultraviolet rays to blue light. Therefore, it was expected that the α-SiAlON phosphor will be used as a yellow-emitting phosphor for white LED since it is allowed to emit a yellow light when it is irradiated with the ultraviolet rays or blue light.
The yellow-emitting phosphor may be prepared by weighing europium oxide and starting materials such as silicon nitride, aluminum nitride and calcium carbonate (CaCO3), all of which are used in the form of powder, mixing certain amounts of the europium oxide and the starting materials, and firing the resulting mixture at high temperature under a nitrogen-containing atmosphere. Also, there have been a proposal for a high-purity raw material in which a content of impurities is stipulated (Patent reference 3), or a proposal for the use of metallic silicon (Patent reference 4) in order to provide high brightness.
Meanwhile, it has been known that β-SiAlON has a crystal structure represented by Formula: Si6-xAlxOxN8-x, and has no large site formed in crystal thereof unlike the α-SiAlON. The patent references 5 and 6 disclose a β-SiAlON phosphor prepared by adding an activator to β-SiAlON. The patent reference 5 proposes a β-SiAlON phosphor using a metal element (i.e. Cu, Ag, or Mn) and a rare earth element (i.e. Eu) as the activator in β-SiAlON. Also, the Eu-activated β-SiAlON phosphors were reported in the patent references 5 and 6, respectively. However, it was reported that the Eu-activated β-SiAlON phosphor described in the patent reference 5 is allowed to emit light at a blue-emitting band of 410 to 440 nm, and the Eu-activated β-SiAlON phosphor described in the patent reference 6 is a green-emitting phosphor. From these results, it was supposed that the difference in emission colors of the Eu-activated β-SiAlON phosphors is derived from the fact that, since Eu-activated β-SiAlON phosphor of the patent reference 5 has a low firing temperature, Eu is not sufficiently dissolved into β-SiAlON, as described above in the patent reference 6.
The Eu-activated β-SiAlON phosphor of the patent reference 6 is characteristic of being exited to emit a green light when it was exposed to the light that is of an ultraviolet ray to a blue light range. Therefore, the Eu-activated β-SiAlON phosphor has received attention as a green-emitting phosphor for white LED that is composed of blue LED and a phosphor, or UV LED and a phosphor. In particular, it is expected that the Eu-activated β-SiAlON phosphor is used as a green-emitting phosphor for white LED requiring high color reproductions since it has a narrow spectrum width of approximately 55 nm and shows its good color purity. However, there is a demand for enhancing brightness of the Eu-activated β-SiAlON phosphor since the Eu-activated β-SiAlON phosphor shows its insufficient brightness.
The β-SiAlON phosphor is prepared by weighing starting materials such as silicon nitride and aluminum nitride and an activator, all of which are used in the form of powder, mixing certain amounts of the starting materials and the activator, and firing the resulting mixture at high temperature in a nitrogen-containing atmosphere. Also, the patent reference 6 discloses a method for preparing a Eu-activated β-SiAlON phosphor. Here, the Eu-activated β-SiAlON phosphor is prepared by weighing starting materials such as silicon nitride and aluminum nitride (or, aluminum oxide) and europium oxide, mixing certain amounts of the starting materials and the europium oxide, and firing the resulting mixture at a high temperature of 1850° C. or above in a nitrogen-containing atmosphere.
As described above, the conventional method, as described in the patent reference 6, using the recently known nitride raw materials such as silicon nitride and aluminum nitride as starting materials has a problem in that it is impossible to obtain a β-SiAlON phosphor having sufficiently high brightness. Also, when the conventional method is used in the field of applications such as white LED, it is necessary to control the particle size distribution such as particle sizes or particle shapes, in addition to the light-emitting characteristics of the β-SiAlON phosphor, so that the particle size distribution can affect luminous efficiency of the white LED device. Also, it is necessary to use a suitable β-SiAlON phosphor for the white LED device since the particle size distribution of the β-SiAlON phosphor affects a manufacturing ratio of the final products.
Furthermore, there are limits on the makers that are able to manufacture silicon nitride and/or aluminum nitride, and therefore kinds of high purity silicon nitride and/or high purity aluminum nitride used as the raw materials are not so much. As a result, there are limitations on the nitride raw materials used, that is, the nitride raw materials having sufficiently high purity are not present in common-grade products and/or the cost of the nitride raw materials is high. That is to say, in the case of the brightness and the particle size distribution depending on the kinds of the used nitride raw materials, the limitations on the nitride raw materials may cause the brightness to be deteriorated and the particle size distribution to be controlled insufficiently.